The .NET Framework 4.6.1 Language Pack contains translated error messages and other UI text for languages other than English. If you don't install a language pack, this text is displayed in English. You can install multiple language packs on one computer, each for a different language.
You need to download and install the .NET Framework 4.6.1 (offline installer) prior to installing the language packs. You can find the .NET Framework 4.6.1 (offline installer) here.